A standard apparatus for drilling the flanges of an angle iron has a transport unit for displacing the workpiece longitudinally through a working station, a support in the station for holding the workpiece with the one flange forming an acute angle with the horizontal, and a drilling unit having a drilling tool. This drilling tool is centered on and rotatable about an axis perpendicular to the flange to be drilled and can be moved axially to drill through the one flange.
It is of course necessary to set the edge-to-center distance, that is the distance between the center of the hole to be drilled and the workpiece edge, very accurately. In many systems in fact the drilling unit is retracted past the workpiece between drilling operations, so such predrilling positioning must be carried out before each drilling operation. Additionally, this distance or spacing not only varies from workpiece to workpiece, as typically the flange is drilled midway between its two edges, but it is also known to drill a staggered line of holes, so that the drilling unit must move laterally relative to the workpiece between successive drilling operations.
Thus it is standard to mount the drilling unit--including a drive motor, chuck, and drill bit--on a carriage that can move in a direction perpendicular to the flange to be drilled, and to mount this carriage in turn on another carriage that can move in a direction that is parallel to the flange to be drilled and that is also perpendicular to the workpiece travel and longitudinal directions. Such an arrangement presents considerable vibration and counterbalancing problems, especially as the drilling unit and the two carriages together are quite heavy. The device tends to wear unevenly, and when imbalanced poses considerably greater loads to the various drives than are necessary.